1. Field of Invention
This invention relates to a method for treatment of exhaust gases and, in particular, to the treatment of gases containing measurable quantities of multivalent heavy metals in high oxidation states to reduce the metals to relatively harmless, lower oxidation states.
2. Brief Statement of the Prior Art
A number of industrial processes utilize multivalent heavy metals in various oxidation states. In these processes, detectable quantities of compounds of the heavy metals can become entrained in the exhaust gases from the industrial processes, usually in solution as droplets, and when the solvent is evaporated from the droplets, as very fine solids. A classic example of a heavy metal is chromium which is present in its hexavalent oxidation state in exhaust gases from chromium plating operations. Chromium plating is used for many applications, including chromium plating for decorative purposes to impart luster and scratch resistance to metal surfaces, as well as to for imparting hardness and wear resistance to mechanical parts. Commonly, chromium is electrolytically plated from an aqueous electrolyte of the salts of this metal. During these plating operations, a detectable quantity of chromium in the hexavalent oxidation state is entrained from the electrolyte in the gases exhausted from chromium plating plants. Although the quantities of chromium which are vaporized in this manner may be exceedingly small, the relative high toxicity of the metal in its hexavalent oxidation state and the great advances which have been made in detection of minute quantities of metals in gases, are resulting in exceedingly stringent standards for permissible levels of this metal.
Although many of the heavy metals, such as chromium, are objectionable in their higher oxidation states, they can be relatively harmless if reduced to lower oxidation states. Such conversion, then, provides a solution to the potential hazard and environmental threat presented by these metals. Accordingly, there is a need for a cost efficient treatment of exhaust gases for the complete reduction of multivalent heavy metals, such as hexavalent chromium, to harmless, lower oxidation states.